In order to reduce the polluting emission, most internal combustion engines, principally diesel engines, are equipped with an exhaust gas recirculation (EGR) system for selectively routing back a part of the exhaust gas from an exhaust manifold of the internal combustion engine into an intake manifold of the internal combustion engine. The recirculated exhaust gas is mixed with fresh induction air and is fed into engine cylinders, thereby reducing the production of oxides of nitrogen (NOx) during the combustion process.
Conventional EGR systems comprise an EGR conduit fluidly connecting the exhaust manifold to the intake manifold, an EGR cooler located in the EGR conduit for cooling the exhaust gas before mixing it with the induction air, and a valve for regulating the flow rate of the exhaust gas to be recirculated in the EGR conduit.
In order to improve temperature control of the recirculated exhaust gas, some EGR systems may comprise two EGR coolers located in series along the EGR conduit. The first EGR cooler is designed to cool the exhaust gas that comes at high temperature (up to about 700°) straight from the exhaust manifold, while the second EGR cooler receives the exhaust gas already partially cooled by the first EGR cooler and is designed to cool the exhaust gas to even lower temperature values (down to about 50° C.). In this way, the exhaust gas is subjected to two cooling stages, including a so called high-temperature cooling stage in the first EGR coolant and a so called low-temperature cooling stage in the second EGR cooler. The first EGR cooler is usually connected with the cooling circuit of the internal combustion engine, so that the high-temperature cooling stage is performed with the aid of the same coolant used for cooling down other engine components, such as engine block and cylinder head. The second EGR cooler is connected to a dedicated coolant circuit so that the high-temperature cooling stage is performed with the aid of a different coolant. A bypass conduit and an associated bypass valve may be provided for the exhaust gas to bypass the second EGR cooler, for example when the value of the exhaust gas temperature is below a predetermined threshold value thereof.
The first and the second EGR cooler may be embodied as a single component, usually referred to as a duel-temperature EGR cooler, which comprises an external casing provided with an inlet and an outlet for the exhaust gas flow and two tube bundles, which are individually provided with an inlet and an outlet for a coolant and which are located inside the external casing so that the exhaust gas flow flows through the tube bundles in series. In this way, the first tube bundle performs the high-temperature cooling stage, whereas the second tube bundle performs the low-temperature cooling stage. The bypass conduit is embodied as a passage defined inside the external casing, alongside the two tube bundles.
Because both tube bundles are located in the same casing but are designed to perform at different temperature values, unwanted exchange of heat between them could reduce the EGR cooler efficiency. Normally the two tube bundles are separated by a separating wall but often this is not enough to prevent unwanted exchange of heat.
Furthermore, during operation, deposits or soot can form on the interior surfaces of the tube bundles causing the EGR cooler to be less efficient and leading to fouling and potential plugging. This phenomenon is more pronounced at lower temperature values making the second tube bundle more sensitive to fouling deterioration.
At least one object herein is to provide an EGR cooler that increases the thermal insulation between the first and the second tube bundles while at the same time helping in the prevention of fouling in the tube bundles and optimizing the layout of the EGR cooler.
Another object herein is to meet these goals by means of a simple, rational and low cost solution. In addition, other objects, desirable features and characteristics will become apparent from the subsequent summary and detailed description, and the appended claims, taken in conjunction with the accompanying drawings and this background.